The present invention relates generally to antenna, and more particularly to a dual mode horn reflector antenna having equalized principle plane beamwidths and reduced sidelobes over a large bandwidth of performance.
Spacecraft communication systems often require medium gain antennas which provide adequate area gains, large bandwidth, equalized principal plane beamwidths and low sidelobes while maintaining a compact physical size. A reflector antenna is typically not a good choice for medium gain applications since the reflector antenna approaches diffraction limitations when used in medium gain applications. A horn antenna is also not a good choice for medium gain applications since the horn becomes excessively large when used in medium to high gain applications. A horn reflector antenna can be a good choice for medium to high gain applications since it is compact in size and can provide approximately medium gain.
A typical horn reflector antenna is comprised of a conical electromagnetic horn and a reflector which is a sector of a paraboloid of revolution, the apex of the horn coincides with the focus of the paraboloid of revolution and the axis of the horn is perpendicular to the axis of the paraboloid. Because of the design, very little energy incident on the reflector is returned into the feed so that mismatch problems are avoided. Also, because of the shielding effect of the horn, side and back lobes are minimized. Therefore, this type of antenna can be quite useful for microwave communications. A more detailed discussion of the conical horn-reflector antenna can be found in "The Electrical Characteristics of the Conical Horn-Reflector Antenna", by J. N. Hines, et. al, published in the Bell System Technical Journal, volume 42, pages 1187-1211, 1963. However, disadvantages in the conical horn-reflector antenna such as unequal principle plane beamwidths and high close in sidelobe levels have prevented the conical horn-reflector antenna from widespread use.
Attempts have been made to equalize the two principle plane beamwidths and reduce the close in sidelobes by using a corrugated horn instead of the typical conical horn. While successful, the machining required to fabricate the corrugated horn section is quite extensive and expensive typically requiring a five axis CNC machine and electroforming the corrugated horn on a mandrel. A more detailed discussion of the corrugated horn-reflector antenna can be found in "The Electrical Characteristics of a Conical Horn-Reflector Antenna Employing a Corrugated Horn", by Ghassan Yassin, et. al., IEEE Transactions on Antennas and Propagation, Vol. 41, No. 3, March 1993.
To avoid the effort and expense involved in manufacturing a corrugate feed while retaining beam symmetry, a Potter horn has been used in a horn-reflector configuration. The Potter horn, detailed in the article "A new horn antenna with suppressed sidelobes and equal beamwidths," by Potter, P. D., Microwave Journal, 4, pg. 71-78, 1963, has a transition section attached to a phasing section feeding a conical horn. An input TE.sub.11 circular mode signal is fed into the transition section and is incident on the phasing section. The phasing section is an axially symmetric step which generates both a TE.sub.11 circular mode signal and a TM.sub.11 circular mode signal form the input TE.sub.11 circular mode signal. The non-symmetric higher order modes are suppressed by the proper choice of the diameter of the phasing section and the asymmetric modes are not strongly excited due to step symmetry. Therefore, only the TE.sub.11 and TM.sub.11 modes propagate after the step. The TE.sub.11 and TM.sub.11 mode signal must be combined in the phasing section in the proper amplitude and phase to produce an antenna pattern having equal principle plane beamwidths. The difference in the diameters before and after the step determines the relative mode amplitude, and length of the phasing section determines the phase. The length of the phasing section is chosen to provide proper combining of the TE.sub.11 and TM.sub.11 mode signals at a design frequency.
As the frequency deviates from the design frequency, the phase relationship between the TE.sub.11 and TM.sub.11 modes changes and proper combining of the two modes no longer occurs generating a non-symmetrical beam pattern. Calculations and measurements have shown that when using the fundamental TE.sub.11 and TM.sub.11 mode signals, a symmetrical pattern results over a relatively narrow bandwidth of approximately 5%.
A need exists to provide a compact antenna that provides medium gain with symmetrical beams over a large bandwidth of operation.